Stay-green is a term used to describe a plant phenotype, e.g., whereby leaf senescence (most easily distinguished by yellowing of the leaf associated with chlorophyll degradation) is delayed compared to a standard reference. See, Thomas H and Howarth C J (2000) “Five ways to stay green” Journal of Experimental Botany 51:329–337. In sorghum, several stay-green genotypes have been identified which exhibit a delay in leaf senescence during grain filling and maturation. See, Duncan R R, et al. (1981) “Descriptive comparison of senescent and non-senescent sorghum genotypes.” Agronomy Journal 73:849–853. Moreover, under conditions of limited water availability, which normally hastens leaf senescence (see, e.g., Rosenow D T, and Clark L E (1981) Drought tolerance in sorghum. In: Loden H D, Wilkinson D, eds. Proceedings of the 36th annual corn and sorghum industry research conference, 18–31), these genotypes retain more green leaf area and continue to fill grain normally (see, e.g., McBee G G, Waskom R M, Miller F R, Creelman R A (1983) Effect of senescence and non-senescence on carbohydrates in sorghum during late kernel maturity states. Crop Science 23:372–377; Rosenow D T, Quisenberry J E, Wendt C W, Clark L E (1983) Drought-tolerant sorghum and cotton germplasm. Agricultural Water Management 7:207–222; and, Borrell A K, Douglas A C L (1996) Maintaining green leaf area in grain sorghum increases yield in a water-limited environment. In: Foale M A, Henzell R G, Kneipp J F, eds. Proceedings of the third Australian sorghum conference. Melbourne: Australian Institute of Agricultural Science, Occasional Publication No. 93). The stay-green phenotype has also been used as a selection criterion for the development of improved varieties of corn, particularly with regard to the development of drought-tolerance. See, e.g., Russell W A (1991) Genetic improvement of maize yields. Advances in Agronomy 46: 245–298; and, Bruce et al. (2002), “Molecular and physiological approaches to maize improvement for drought tolerance” Journal of Experimental Botany, 53 (366): 13–25.
Five fundamentally distinct types of stay-green have been described, which are Types A, B, C, D and E (see e.g., Thomas H, Smart C M (1993) Crops that stay green. Annals of Applied Biology 123:193–219; and, Thomas and Howarth, supra). In Type A stay-green, initiation of the senescence program is delayed, but then proceeds at a normal rate. In Type B stay-green, while initiation of the senescence program is unchanged, the progression is comparatively slower. In Type C stay-green, chlorophyll is retained even though senescence (as determined through measurements of physiological function such as photosynthetic capacity) proceeds at a normal rate. Type D stay-green is more artificial in that killing of the leaf (i.e. by freezing, boiling or drying) prevents initiation of the senescence program, thereby stopping the degradation of chlorophyll. In Type E stay-green, initial levels of chlorophyll are higher, while initiation and progression of leaf senescence are unchanged, thereby giving the illusion of a relatively slower progression rate. Type A and B are functional stay-greens, as photosynthetic capacity is maintained along with chlorophyll content, and these are the types associated with increased yield and drought tolerance in sorghum. Despite the potential importance of this trait, in particular the benefits associated with increasing yield and drought tolerance, very little progress has been made in understanding the biochemical, physiological or molecular basis for genetically determined stay-green (Thomas and Howarth, supra).
This invention solves these and other problems. The invention relates to the identification of ACC synthase genes associated with staygreen potential phenotype in plants and modulation of staygreen potential and/or ethylene production. Polypeptides encoded by these genes, methods for modulating staygreen potential in plants, methods for inhibiting ethylene production in plants, methods for modulating sterility in plants, and knockout plant cells and plants, as well as other features, will become apparent upon review of the following materials.